1. Field of Invention
This invention relates to laser radar systems (ladars). Specifically, the present invention relates to synthetic aperture ladar systems employing incoherent laser pulses.
2. Description of the Related Art
Ladar systems are employed in various applications including high-resolution 3-dimensional imaging, mapping, chemical analysis, and military targeting applications. Such applications require accurate, space-efficient, and cost-effective ladar systems.
Ladar systems are particularly applicable for long-range, high-resolution 3-dimensional imaging applications employed in terrain mapping and target imaging applications on satellites and missile systems. A ladar system often includes a sensor suite mounted on a satellite, missile system, or aircraft. The sensor suite has one or more fixed physical apertures through which a ladar system views a scene. A ladar system views a scene by transmitting a laser through the aperture toward the scene. The laser reflects off the scene, producing a laser return that is detected by the ladar system. Many conventional radar and ladar systems measure the intensity of the return beam and the round trip delay from transmission to detection, which yields the distance (range) to the scene. Laser return intensity and range information may be combined with other image information to facilitate target tracking, terrain mapping, and so on.
Ladar systems are either coherent or noncoherent. Coherent ladar systems transmit a laser beam with a predetermined phase and frequency. Knowledge of the spectral characteristics of the transmitted laser beam enables coherent ladar systems to record additional information about the scene, such as target movement, and to further improve Signal-to-Noise Ratio (SNR) over corresponding noncoherent ladar systems. The velocity of a target may be determined from the frequency spectrum of the laser return.
Conventional noncoherent ladar systems typically lack phase and frequency information pertaining to the transmitted laser beam. A noncoherent detector combines various wavelengths of the laser return and converts them into corresponding electrical signals. Consequently, without laser spectrum information, certain types of noise filtering, which would increase SNR, are difficult or impossible to implement.
Generally, coherent ladar systems have several advantages over noncoherent ladar systems. For example, coherent ladar systems generally have better SNR""s than corresponding noncoherent systems. Unlike incoherent ladar systems, coherent ladar systems may reach Shot Noise Limited (SNL) sensitivity to maximize the SNR. SNL sensitivity is achieved by scaling up the power of a local oscillator aimed on the detector surfaces.
Typically, a coherent ladar system receiver detector is illuminated by a laser return and a local oscillator reference beam. The detector outputs a cross-product of the laser return and local oscillator optical fields. The desired information about a scene is contained in the portion of the detector""s output that oscillates at the frequency difference between the local oscillator reference beam and the laser return. This output is often narrow-band filtered to eliminate noise in frequency regions outside predicted signal locations. This noise filtering is enabled by the preservation of the spectrum information pertaining of the transmit laser by an optical heterodyne or homodyne detection process. Noncoherent ladar systems generally do not perform this noise filtering, since they lack requisite spectrum information pertaining to the transmitted laser beam. Unfortunately, coherent ladar systems are generally more sensitive to misalignments and beam distortions.
In a conventional ladar imaging system not employing synthetic aperture methods, image cross-resolution is limited by the size of the ladar system aperture. Very large and expensive apertures are required to obtain sufficient resolution for many current long-range imaging and mapping applications. This is particularly problematic for ladar systems employed in satellites or missile systems, which have prohibitive space constraints and require long-range viewing capabilities.
To reduce aperture-size requirements, synthetic aperture radar and ladar systems are employed. In a synthetic aperture ladar system, additional information about the scene is obtained by changing the viewing angle of the scene. This additional information, called cross-range information, is contained in Doppler frequency shifts detected in the laser return caused by the transmit laser striking various features of the scene at different angles. Cross-range information indicates the relative angular position of certain scene features associated with a given range or distance from the ladar system. The cross-range information is combined with range information to yield an accurate scene profile to enhance the image of the scene.
High resolution topography applications operating at a range of approximately 100 kilometers, an eye-safe laser wavelength of 1.5xc3x9710xe2x88x926 m, and a typical cross resolution of 20 cm, require a conventional aperture of approximately 75 cm, which is prohibitively large and expensive for many applications. The large apertures are also undesirably sensitive to thermal and gravitational distortions. An analogous synthetic aperture ladar system on a platform travelling at, for example, 100 m/s would require 7.5 milliseconds (ms) to cover the required 75 cm aperture. In traditional ladar, this requires that the laser transmitter produce a high-power waveform that is coherent for the full 7.5 ms. The high power is often required to reach long ranges of interest. Typically, coherent waveforms longer than a fraction of a millisecond are difficult to achieve, especially at high power levels. In addition to coherence time and high power, the transmitted waveform requires high bandwidth to achieve high down-range resolution, yielding typical bandwidth-time products (BT) greater than 300,000. This implies that the transmitted waveform must be accurate (phase coherent) to {fraction (1/300,000)} (1/BT). Consequently, conventional synthetic aperture ladar systems have generally been unsuccessful in achieving this bandwidth time product.
Previous synthetic aperture ladar systems could not maintain transmitter coherence for sufficient duration to accurately measure a scene. Accurate synthetic aperture measurements require relatively high beam pulse energy for which coherence is difficult to maintain. For example, synthetic aperture ladar systems employing trains of FM chirped signals are employed on some mobile ladar systems. Unfortunately, these systems have difficulty maintaining laser beam coherence, yielding inferior imaging capabilities.
Generally, conventional synthetic aperture ladar systems require a coherent waveform throughout the measuring time during which the laser return is detected. This severely limits waveform selection, preventing use of otherwise more desirable waveforms, such as high-energy Q-switched pulses.
Hence, a need exists in the art for an efficient synthetic aperture ladar system that does not require transmission of a coherent laser beam yet maintains the advantages of coherent ladar systems over those of conventional noncoherent ladar systems while maintaining beam alignment advantages of noncoherent systems. There exists a further need for a synthetic aperture ladar system that employs Q-switched laser pulses and an accompanying receiver for detecting a Q-switched laser return.
The need in the art is addressed by the synthetic aperture ladar system using incoherent laser pulses of the present invention. In the illustrative embodiment, the inventive system is adapted for use with synthetic aperture ladar systems employed in military targeting and imaging applications. The ladar system includes a first mechanism for generating a laser beam. A second mechanism records phase information pertaining to the laser beam and subsequently transmits the laser beam from the system in response thereto. A third mechanism receives a reflected version of the laser beam and provides a received signal in response thereto. A fourth mechanism corrects the received signal based on the phase information recorded by the second mechanism.
In a specific embodiment, the ladar system is a synthetic aperture ladar system that further includes a fifth mechanism for moving the ladar system while the ladar system operates. The fourth mechanism includes a synthetic aperture processor for correcting the received signal in accordance with the phase information and providing a corrected ladar signal in response thereto. The synthetic aperture processor includes a mechanism for applying a Discrete Fourier Transform (DFT) to the corrected ladar signal to obtain high-resolution frequency and cross-range information. A fifth mechanism constructs a coherent range-Doppler scene profile based on the corrected ladar signal and the movement of the ladar system.
The first mechanism includes an Er:Yb:Glass Q-switched laser or an Er:Yb:YAG high-power laser for generating the transmitted laser beam. The second mechanism includes a digitizer for recording the phase information and frequency information. The phase information includes waveform information about the transmitted laser beam including measured phase jumps, phase offsets, frequency hops, and frequency offsets. The transmitted laser beam comprises Q-switched or Q-switched mode locked ladar pulses having random phase (incoherent) from shot to shot.
The third mechanism includes an In-phase (I) and Quadrature (Q) receiver for implementing I and Q detection and outputting the received signal having I and Q electrical signal components in response thereto. The I and Q receiver is an optical heterodyne receiver that includes a local oscillator for generating reference beam. An optical retarder shifts the reference beam. An l-detector and a Q-detector detect a combination of the reference beam and the reflected version of the laser beam and a combination of the shifted reference beam and the reflected version of the laser beam, respectively. The I and Q heterodyne receiver further includes one or more beam splitters having reflectivities specified to equalize intensities of the reflected version of the laser beam, the reference beam, and the shifted reference beam at the I and Q detectors.
In a more specific embodiment, the third mechanism further includes a digitizer for converting the received signal from an analog signal to a digital received signal with I and Q components. A range demultiplexer isolates portions ((rl+i*rQ)n,) of the digital received signal which represent laser returns, each associated with a range bin (n).
The fourth mechanism maintains detected phases (xcex81, xcex82, xcex8m, . . . xcex8M) and frequency offset (f1, f2, fm, . . . fM) associated with each of the M transmitted laser pulses. Another mechanism corrects the digital received signal ((rl+i*rQ)n,m) based on the detected phases and frequency offsets that were measured on the outgoing pulses and provides the corrected signal in response thereto in accordance with the following, equation:
Corrected Signal=Re{(rl+ixc2x7rQn,mxc2x7e(xe2x88x92i(xcex8m+2xcfx80fmxcfx84))
where (rl+i*rQ)n,m represents a portion of the digital received signal associated with an nth range bin and the mth pulse having an in-phase component rl and a quadrature component rQ; xcex8m represents a phase correction term associated with one of the detected phases that is associated with the mth pulse; fm represents a frequency correction term associated with the mth pulse; and xcfx84 is a digital time variable.
The fourth mechanism further includes a mechanism for computing centroids, one centroid for each nth portion of the received digital signal, based on the square of the magnitude of a DFT of each nth portion of the received digital. Another mechanism extracts peak intensity information and range Doppler information from the centroids and image information about the scene.
The novel design of the present invention is facilitated by the second mechanism for recording phase information about the transmitted laser beam and by the fourth mechanism for correcting the laser return based on the recorded phase information. This relieves coherence requirements on the transmitted laser, thereby enabling use of very desirable transmit waveforms, such as high-energy Q-switched beams, for which coherence is difficult to maintain.